1. Field of the Invention
The present invention relates to a pattern defect correction method of a photomask, and more particularly, to a method of correcting in high precision by a focused ion beam (referred to as FIB hereinafter) an opaque defect (pattern residue) and the like of a phase shift pattern generated informing a phase shift mask used in manufacturing an LSI.
2. Description of the Background Art
In manufacturing a semiconductor, the resolution R (.mu.m) of lithography technique used in patterning interconnections is represented as: EQU R=k.sub.1 .lambda./NA, k.sub.1 =0.5
where .lambda. is the light wavelength (.mu.m) used for exposure, and NA is the numerical aperture of the lens. According to the above equation, the resolution of photolithography is decreased as the light wavelength becomes smaller, and also if the value of NA increases or if constant k.sub.1 depending on the resist decreases. Using an i-line as the light for exposure (.lambda.=0.365 .mu.m) with numerical aperture NA=0.5 and constant k.sub.1 =0.5, a resolution of approximately 0.4 .mu.m is obtained. In order to improve the resolution, the light wavelength must be reduced or the value of the numerical aperture NA be increased. However, it is technically difficult to obtain a photolens meeting such conditions, and the depth of focus .delta. (=.lambda./2NA.sup.2) will be reduced.
To overcome such problems, a phase shift exposure method is proposed in, for example, Japanese Patent Laying-Open No. 57-62052 and Japanese Patent Laying-Open No. 58-173744.
A conventional photomask that does not shift the phase, and a conventional photomask using a phase shift mask will be described hereinafter with reference to FIGS. 13A-13C and FIGS. 14A-14C.
In a conventional photomask which does not shift the phase, the electric field of light passing between the mask patterns 2 formed on a mask substrate 1 (FIG. 13A) is spatially separated from each other as shown in FIG. 13B. However it is not possible to focus an image of the mask patterns because the light intensity is distributed continuously as shown in FIG. 13C.
In the case of using a phase shift film, a phase shift pattern 3 formed of a SiO.sub.2 film or the like is provided at every other portion between neighboring mask patterns 2, as shown in FIG. 14A. The phase of light passing between the mask patterns 2 will be displaced in phase by 180 degrees, so that the patterns of the electric field on the mask is distributed in inversion at every other pattern, as shown in FIG. 14B. Therefore, the light intensity on the mask shows a separated pattern, as shown in FIG. 14C. By using a phase shift mask with the above-described mechanism, the resolution can be reduced to approximately a half of the pattern width in comparison with the case of a photomask that does not shift the phase.
A method of correcting an opaque defect of a photomask using a conventional phase shift mask will be described hereinafter with reference to FIG. 15 and FIGS. 16A-16F.
Referring to FIG. 15, an opaque defect 13 is generated due to the material of a phase shifter remaining in a region between light shielding films 12 formed of a metal such as Cr or an intermetallic compound such as MoSi and not covered with the phase shifter on a mask substrate 11. The removal of the opaque defect 13 was carried out conventionally as shown in the steps of FIGS. 16A-16C and FIGS. 16D-16F. More specifically, an FIB is directed to scan only the region in the proximity of the opaque defect (the region XVIc in FIG. 15) to remove the opaque defect 13 by etching (FIG. 16B; FIG. 16E). When the etching process is stopped at the time the opaque defect 13 is completely removed, a recess 13a running along the configuration of the opaque defect 13 is seen due to the surface of the mask substrate 11 being overetched. The generation of such a recess 13a along the configuration of the opaque defect 13 on the surface of the mask substrate 11 is caused by the fact that the quartz used as the material of the mask substrate (11) and the SOG (Spin On Glass) used as the material of the phase shifter forming the opaque defect substantially have the same etching rate.
Because the correction method of a pattern defect of a photomask having a phase shift pattern was carried out as described above, a portion of the mask substrate was etched to result in a recess portion affected by the profile of the opaque defect in the defect region after the correction process. This will degrade the performance of the photomask.